Aim: The specificity of muscle‐tendon and foot architecture of elite Kenyan middle‐ and long‐distance runners has been found to contribute to their superior running performance. To investigate the respective influence of genetic endowment and training on these characteristics, we compared leg and foot segmental lengths as well as muscle‐tendon architecture of Kenyans and Japanese males (i) from infancy to adulthood and (ii) non‐athletes versus elite runners.Methods: The 676 participants were divided according to their nationality (Kenyans and Japanese), age (nine different age groups for non‐athletes) and performance level in middle‐ and long‐distance races (non‐athlete, non‐elite and elite adult runners). Shank and Achilles tendon (AT) lengths, medial gastrocnemius (MG) fascicle length, pennation angle and muscle thickness, AT moment arm (MA_AT), and foot lever ratio were measured.Results: Above 8 years old, Kenyans had a longer shank and AT, shorter fascicle, greater pennation angle, thinner MG muscle as well as longer MA_AT, with lower foot lever ratio than age‐matched Japanese. Among adults of different performance levels and independently of the performance level, Kenyans had longer shank, AT and MA_AT, thinner MG muscle thickness, and lower foot lever ratio than Japanese. The decrease in MG fascicle length and increase pennation angle observed for the adult Japanese with the increase in performance level resulted in a lack of difference between elite Kenyans and Japanese.Conclusion: The specificity of muscle‐tendon and foot architecture of elite Kenyan runners could result from genetic endowment and contribute to the dominance of Kenyans in middle‐ and long‐distance races.

This study aimed to compare different ultrasound devices with magnetic resonance spectroscopy (MRS) to quantify muscle lipid content from echo intensity (EI). Four different ultrasound devices were used to measure muscle EI and subcutaneous fat thickness in four lower-limb muscles. Intramuscular fat (IMF), intramyocellular (IMCL) and extramyocellular lipids (EMCL) were measured using MRS. Linear regression was used to compare raw and subcutaneous fat thickness-corrected EI values to IMCL, EMCL and IMF. IMCL had a poor correlation with muscle EI (r = 0.17–0.32, NS), while EMCL (r = 0.41–0.84, p < 0.05–p < 0.001) and IMF (r = 0.49–0.84, p < 0.01–p < 0.001) had moderate to strong correlation with raw EI. All relationships were improved when considering the effect of subcutaneous fat thickness on muscle EI measurements. The slopes of the relationships were similar across devices, but there were some differences in the y-intercepts when raw EI values were used. These differences disappeared when subcutaneous fat thickness-corrected EI values were considered, allowing for the creation of generic prediction equations (r = 0.41–0.68, p < 0.001). These equations can be used to quantify IMF and EMCL within lower limb muscles from corrected-EI values in non-obese subjects, regardless of the ultrasound device used.

This study investigated the sex influence on the acute and delayed fatigue effects of a 20 km graded running race. Eighteen recreational runners, 10 women and 8 men, completed the race. The testing protocol included five sessions: a week before the race (PRE), 35 ± 15 min after (POST), 2 h, 2 and 4 days (2D and 4D) later. Each session included uni- and bilateral maximal isometric voluntary contractions of the knee extensors (MVC), a squat jump (SJ), and a drop jump (DJ). Acute and delayed muscle soreness (DOMS) were evaluated for the quadriceps, hamstring and triceps surae muscle groups. The 2D and 4D sessions included also a horizontal force-velocity test (HF-V) performed under five resistive conditions. For each test, a set of key variables was computed to characterize the lower limb functional recovery. Mixed ANOVA analyses revealed significant (sex × time) interactions, with larger acute drops for men in MVCs and earlier recovery for women in the bilateral MVC (p< 0.001) and DJ (p< 0.05) tests. Only women reported DOMS for the hamstrings at 2D (p< 0.001) and showed small improvements in pure concentric SJ (p< 0.05) and HF-V (p< 0.01) tests at 4D. As expected, DOMS disappeared prior to the complete functional recovery. These results confirmed the combined influence of testing task and sex on the functional recovery pattern while supporting a lesser and faster recovery in women. The originality of this study lies in the complexity and sex-dependence of the functional recovery pattern revealed by a multiple factorial analysis which was used to identify the most discriminating tests and variables in the recovery pattern. The obtained clusters highlighted some recovery profiles associated with greater risks of injury when starting to run again. However, the lack of sex × time interaction for normalized values emphasizes the major influence of men’s initially higher functional values compared to women.

Walking and running are mechanically and energetically different locomotion modes. For selecting one or another, speed is a parameter of paramount importance. Yet, both are likely controlled by similar low-dimensional neuronal networks that reflect in patterned muscle activations called muscle synergies. Here, we investigated how humans synergistically activate muscles during locomotion at different submaximal and maximal speeds. We analysed the duration and complexity (or irregularity) over time of motor primitives, the temporal components of muscle synergies. We found that the challenge imposed by controlling high-speed locomotion forces the central nervous system to produce muscle activation patterns that are wider and less complex relative to the duration of the gait cycle. The motor modules, or time-independent coefficients, were redistributed as locomotion speed changed. These outcomes show that robust locomotion control at challenging speeds is achieved by modulating the relative contribution of muscle activations and producing less complex and wider control signals, whereas slow speeds allow for more irregular control.

Background: In endurance running, elite Kenyan runners are characterized by longer thigh, shank, and Achilles tendon (AT) lengths combined with shorter fascicles and larger medial gastrocnemius (MG) pennation angles than elite Japanese runners. These muscle-tendon characteristics may contribute to the running performance of Kenyans. Furthermore, these specific lower-leg musculoskeletal architectures have been confirmed not only in elite Kenyan runners but also in non-athletic Kenyans since early childhood. However, it remains questionable whether the differences in muscle-tendon architecture between Kenyans and Japanese differ from those of European Caucasians. Therefore, this study aimed to compare anthropometry and muscle–tendon architecture of young non-athletic Kenyan males with their Japanese and French counterparts. Methods: A total of 235 young non-athletic males, aged 17–22 years, volunteered. The anthropometric measures, thigh, and shank lengths, as well as AT and MG muscle architecture, were measured using ultrasonography and a tape measure. Inter-group differences in anthropometry and muscle-tendon architecture were tested using one-way ANOVA and ANCOVA analyses controlling for shank length and muscle thickness. Results: The anthropometric and muscle-tendon characteristics of the non-athletic French were closer to those of the Kenyans than to those of the Japanese. However, the ultrasonography analysis confirmed that the non-athletic Kenyans had the longest AT as well as the shortest MG fascicles and the largest pennation angle compared to the French and Japanese, even after controlling for shank length and muscle thickness with ANCOVA, respectively. Conclusions: These results confirmed the specificity of the muscle-tendon architecture of the triceps surae in Kenyans in comparison to their Japanese and French counterparts in non-athletic adults. This study provides additional support to the fact that Kenyans may have musculotendinous advantages in endurance running.